1. Field of the Invention
The present invention relates to an optical system. More specifically, the present invention relates to an optical system useful in an optical apparatus, such as a silver-halide film camera, a digital still camera, a video camera, a digital video camera, a telescope, binoculars, a projector, or a copying machine.
2. Description of the Related Art
It is desired by the market that an optical system used in an optical apparatus, such as a digital camera, a video camera, or a projector, has a wide angle of view and is capable of forming a high-quality image (a projection image if a projector is used) without causing an aberration such as distortion.
As an optical system whose angle of view and back focal length can be easily enlarged or increased, a retrofocus type optical system is widely used. A retrofocus type optical system is widely used as a photographic optical system for an image pickup apparatus and a projection optical system for a projection apparatus (projector).
In a retrofocus type optical system, a lens unit having an overall negative refractive power is located on the front side of the optical system (on the object side in a photographic optical system and on the screen side in a projection optical system). On the rear side of the optical system (on the image side in a photographic optical system and on the original image side in a projection optical system), a lens unit having a positive refractive power is located.
Japanese Patent Application Laid-Open No. 09-033801, U.S. Pat. No. 7,426,079, and Japanese Patent Application Laid-Open No. 2001-188171 each discuss a retrofocus type photographic optical system including, in order from the object side to the image side, a first lens unit having a negative refractive power and a second lens unit having a positive refractive power.
Japanese Patent Application Laid-Open No. 06-082689 and U.S. Pat. No. 7,079,324 each discuss a retrofocus type projection optical system including, in order from the screen side to the original image side (the side of an image to be projected), a first lens unit having a negative refractive power and a second lens unit having a positive refractive power.
In a retrofocus type optical system, a lens unit having a negative refractive power is located ahead of other lens units. Accordingly, in a retrofocus type optical system, the arrangement of refractive powers is asymmetric with respect to the aperture stop. Therefore, negative distortion (barrel distortion) may easy occur.
In order to reduce negative distortion, it is useful to use a material having a high refractive index as a material of a negative lens included in the lens unit having a negative refractive power. However, a material having a high refractive index generally has a high dispersion. Accordingly, if a material having a high refractive index is used as a material of a negative lens included in a lens unit having a negative refractive power, a large amount of negative chromatic aberration of magnification (lateral chromatic aberration) may occur.
In order to correct chromatic aberration of magnification that may occur in a retrofocus type photographic optical system, it is useful to locate a positive lens made of a high-dispersion glass material within a first lens unit having a negative refractive power or a lens unit located in proximity to the first lens unit.
In addition, in order to correct negative chromatic aberration of magnification that may occur in a retrofocus type photographic optical system, it is useful to use a positive lens made of a low-dispersion material having an anomalous partial dispersion characteristic in a lens unit located closer to the rear side of the optical system than an aperture stop, whose height h of incidence of a paraxial chief ray (the distance from the optical axis) on a lens surface is relatively high.
However, with any of the above-described conventional methods, it is likely that some chromatic aberration of magnification may remain uncorrected because high-order chromatic aberration of magnification cannot be effectively corrected in this case. Accordingly, as the image height increases, chromatic aberration of magnification with respect to g-line light may increase in the positive direction. Therefore, the entire image is likely to be overcorrected.
In a conventional retrofocus type optical system, in order to balance chromatic aberration of magnification over the entire image plane, correction of aberration is executed so that chromatic aberration of magnification becomes negative at a medium image height and positive at a maximum image height.
Furthermore, a conventional retrofocus type optical system corrects aberration by using a high-dispersion plastic material having anomalous partial dispersion as a material of a lens of a lens unit, which is located closer to the rear side of the optical system than the aperture stop and whose height h of incidence of a paraxial chief ray on a surface of the lens (the distance from the optical axis) is relatively high.
However, if a plastic material is used as a lens material, one single lens cannot have a large power. Accordingly, the capacity of the lens to correct monochromatic aberration becomes small. In addition, if the refractive power of the lens is increased, when the refractive index of the material is low, various aberrations, such as distortion for a reference wavelength, may remain uncorrected.